Not sure why forced confessions are being brought up here? Most heroes generally aren't trying to score a airtight criminal case. They're trying to scare criminals straight. If they were overly concerned with follow the law, they wouldn't be costumed vigilantes in the first place. They'd be cops, fire fighters, or SHIELD agents. Let's not forget, the very act of being a costumed mystery man is all about not having faith in the system and going outside of it to accomplish their goals.
And that's assuming the law in the MU is the same as the law in the real world. The MU could, and almost certainly does, have different policies for this sort of thing. Didn't a arc in Daredevil deal with this not too long ago?